1. Field of the Invention
The present invention relates to measurement of distortion of an optical fiber.
2. Description of the Prior Art
Conventionally, Brillouin scattered light is coherently detected by supplying an optical fiber with pulsed light generated by pulsing continuous wave light to acquire Brillouin scattered light from the optical fiber (refer to FIG. 8 of Japanese Laid-Open Patent Publication (Kokai) No. 2001-165808). The coherent detection is carried out by multiplexing the Brillouin scattered light and intensity-modulated light obtained by applying intensity modulation to the continuous wave light at a predetermined frequency, for example. It should be noted that the intensity-modulated light includes a carrier light component and side band light components of optical frequency.
A signal corresponding to the Brillouin scattered light is extracted by a filter form a result of the coherent detection thereby obtaining a power spectrum of the Brillouin scattered light. It should be noted that the power spectrum of the Brillouin scattered light is acquired while the predetermined frequency is being changed. Moreover, a peak frequency where the power of the Brillouin scattered light takes the maximum value is acquired by fitting a predetermined function (such as a Lorentzian function) to the power spectrum of Brillouin scattered light. A value of distortion of the optical fiber is obtained based on the peak frequency.
FIG. 12 shows an example of a distortion distribution of an optical fiber FIG. 12 is a chart associating the distortion with the distance from an incident end of the optical fiber according to prior art. According to the distortion distribution shown in FIG. 12, the optical fiber has a point A presenting a small distortion, and a point B presenting a large distortion. When pulsed light is supplied to the optical fiber having the distortion distribution shown in FIG. 12, there is obtained a power spectrum of Brillouin scattered light as shown in FIG. 13.
FIG. 13 is a chart associating the optical power with the optical frequency of the Brillouin scattered light according to prior art. As shown in FIG. 13, there is obtained a power spectrum “a” due to the point A and a power spectrum “b” due to the point B. In order to measure the distortion at the point B, it is necessary to acquire a peak frequency fb of the power spectrum “b”.
However it is difficult to precisely acquire the peak frequency fb. FIG. 14 shows an actually-measured peak frequency fp acquired by fitting the predetermined function (such as a Lorentzian function) to the power spectrum of the Brillouin scattered light according to the prior art shown in FIG. 13.
In FIG. 13, the power spectrum is shown as a solid line. However, the data of the power spectrum is actually discrete as shown in FIG. 14, and thus requires fitting. The result of the fitting to the power spectrum is influenced by the power spectrum “a” in addition to the power spectrum “b”. As a result, the actually-measured peak frequency fp where the result of the fitting takes the peak is displaced from the peak frequency fb. It is thus difficult to precisely measure the distortion of the optical fiber at the point B.